add hakka sandhi

src/libespeak-ng/intonation.c

 		if (p->synthflags & SFLAG_SYLLABLE) {
 			tone_ph = p->tone_ph;
 			tph = phoneme_tab[tone_ph];
-
+			// Hakka
+			if (tr->translator_name == L3('h','a','k')){
+				if (prev_tph->mnemonic == 0x31){ // [previous one is 1st tone]
+				  // [this one is 1st, 4th, or 6th tone]
+				  if (tph->mnemonic == 0x31 || tph->mnemonic == 0x34 ||
+					  tph->mnemonic == 0x36){
+					prev_p->tone_ph = PhonemeCode('2');
+				  }
+				}
+			  }
 			// Mandarin
 			if (tr->translator_name == L('z', 'h')) {
 				if (tone_ph == 0) {